Silver bromide and silver bromoiodide emulsions, hereinafter collectively referred to as silver brom(oiod)ide emulsions, typically exhibit regular or irregular octahedral grain shapes. That is, most if not all of the exterior surface area of the grains is accounted for by {111} crystal faces. The art has adopted the practice of referring to {111} crystal faces octahedral faces, since regular grains with {111} crystal faces take the shape of a regular octahedron.
Silver brom(oiod)ide emulsions possess native imaging sensitivity in the ultraviolet and blue portions of the electromagnetic spectrum. Spectral sensitizing dyes have been developed to extend the imaging response of silver brom(oiod)ide throughout the visible spectrum.
One of the art recognized problems in sensitizing emulsions to regions of the spectrum to which they lack native sensitivity is dye desensitization. Notwithstanding the general recognition of dye desensitization as a problem by those skilled in the art, some elaboration is offered, since it is not intuitively obvious that a silver halide emulsion that shows no response to exposure in a spectral region to which the grains possess no native sensitivity in the absence of a spectral sensitizing dye, but responds in the presence of the dye, has been desensitized. Mees, The Theory of the Photographic Process, 3rd Ed., Macmillan, 1966, at page 257, explains dye desensitization a and its verification. When silver halide grains are chemically sensitized, the speed of the emulsion is increased at all wavelengths. Other materials placed in or on the grains desensitize the emulsion at all wavelengths and are referred to as desensitizers. Spectral sensitizing dyes extend the sensitivity of the grains to wavelengths to which the grains lack native sensitivity, but often additionally reduce the sensitivity of the grains in the spectral region of native sensitivity. The reduction of sensitivity imparted by the dye provides an indirect indication that the dye is also reducing sensitivity in the region of spectral sensitization. The generally accepted theory stated by Mees and indicated to be consistent with results obtained by its application is that at any instant of exposure, only a minute fraction of the dye molecules on any grain are in the excited state, with the remaining, unexcited dye molecules remaining capable of adversely affecting grain sensitivity independently of the excited molecules.
Marchetti et al U.S. Pat. No. 4,937,180 recognized that formation of silver brom(oiod)ide grains in the presence of a hexacoordination complex of rhenium, ruthenium, or osmium with at least four cyanide ligands would increase the stability of the emulsions and reduce low intensity reciprocity failure. Marchetti et al recognized that the cyanide ligands were incorporated in the grain structure.
Shiba et al U.S. Pat. No. 3,790,390, Ohkubo et al U.S. Pat. No. 3,890,154, and Habu et al U.S. Pat. No. 4,147,542 disclose emulsions particularly adapted to imaging flash (less than 10.sup.-5 second) exposures. Polymethine cyanine and merocyanine dyes are disclosed having up to three methine groups joining their nuclei with blue flash exposures being suggested with zero, one or two methine linking groups and green flash exposures being suggested with three methine linking groups. In addition to the dyes it is suggested to incorporate in the emulsions compounds of Group VIII metals--i.e., iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. Iron compounds suggested for incorporation are ferrous sulfate, ferric chloride, potassium hexacyanoferrate (II) or (III), and ferricyanide. Shiba et al, Ohkubo et al, and Habu et al suggest incorporation of the iron compounds at any convenient stage from precipitation to coating, indicating that whether the iron is located within or exterior of the grains is inconsequential to the utility taught.